Taylor



Q M A L- w)" Aug. 6, 1957 Filed Jan. 20, 1954 HELJ C AL. RESISTANCEELEME NT R. TAYLOR 2,801,806

APPARATUS FOR WINDING LOAD COMPENSATED POTENTIOMETERS 5 Sheets-Sheet 1 22 MOTOR V GEN.

CHOPPER g I DIFFERENTIAL I I I l 4 3 $2 M 5% :U 0.0) 8 1% v Q IN V ENTOR RICHARD TAYLOR 3% ATTORNEY Aug. 6, 1957 R. TAYLOR 2,801,806

APPARATUS FOR WINDING LOAD COMPENSATED POTENTIOMETERS Filed Jan. 20,1954 s Sheets-Sheet 2 m -mubm. 023% n somlm IN V EN TOR RICHARD TAYLORAT TORNZY Aug. 6, 1957 R. TAYLOR 2,801,806 APPARATUS FOR WINDING LOADCOMP ENSATED PQTENTIOMETERS Filed Jan. 20, 1954 3 Sheets-Sheet 3 IN V ENTOR RICHARD TAYLOR AT TORN United States Patent APPARATUS FOR WINDINGLOAD COMPEN- SATED POTENTIOMETERS Richard Taylor, Binghamton, N. :Y.,assignor to International Business Machines Corporation, New York, N.Y}, a corporation or New York Application llanuary 20, 1954, Serial No.405,173 Claims. c1. 24'2==-9 The present invention relates towindin'ga'pparatus and more particularly to an apparatus for windingpotentiom ete'rs which are automatically compensated for a prescribedload impedance.

An object of this invention is to provide an improved winding apparatus.v ,7

Another object of this inventicn is to provide an improved Windingapparatus for winding potentiometers and the like.

Another object of the present invention is to furnish an improvedWinding apparatus 'for' roducing a po'ten tio'meter having a non-linearresistance characteristic.

Still another object of the invention is to supply an impi'oved windingapparatus for winding potentiomete'rs which are automatically comensated fer a particular load impedance to which the potentiometer isadapted to be connected.

Another object of this invention is the provision of apparatus forwinding a resistance element in the form or a series of elasticsub-portions on a form in a manner to rovide a potentiometer 'whos'eoutpnt precisely follows a prescribed function when connected to aparticular load impedance.

-A still further object or the invention is to provide a potentiometerWinding apparatus for winding a resistance element in the from of aseries of turns on a winding form wherein control means is furnished forautomatically determining the rate at which the turns the to be wound,said rate being a functidn 6f the load to which the potentio'm'eter isadapted to be connected.

Another object of the invention is to provide improved winding apparatusfor producing a potentiometer from a reference potential source whichfollows a 'prescribed function, the resistance of said potentiometerfollowing a second function at variance with said predetermined functionbut the potential output from said potentiometer following saidpredetermined function when connected to a predetermined load impedance.

Other objects of the invention will be pointed out in the followingdescription and claims and illustrated in the accompanying drawings,which disclose, by way of examples, the principle of the invention andthe best mode, which has been contemplated, of suppl ing that principle.

In the drawings:

Fig. 1 is an electromechanical schematic diagram of the windingapparatus; 4

Fig. 2 is an electrical schematic diagram or the winding apparatus;

Fig. 3 is a schematic diagram 'of a typical servoampliher which may beused with the winding apparatus;

Fig. 4 shows an enlarged section of a number of the turns making up theresistance element and the contact means for engaging the coils;

Fig. 5 is an elevation of the potentiometer in a finished state;

Fig. 6 is a view taken on line of Fig. 5, parts of the view being brokenaway for greater detail; and

Fig. 7 compares the characteristic c'iifve of the master "ice with thecharacteristic curves of various Potentiometers compensated fordifferent load ir'npedances.

Similar reference characters represent similar parts throughout theseveral views.

The manufacture of high precision otentiometers such as those used inmodern analog computers is extremely difiicult, particularly where thetolerances are quite small. These potentiometers are usually of themultiple-turn helical type and often have an over-all linearityrequirement of less than .02 percent error.

At present, one method of manufacturing precision potentiometers is toaccurately wind 'the resistance wire on a mandrel in the amount desired.The spacing of the wi're on the mandrel may be accomplished throughlathe feeding means. The mandrel is then wound around the potentiometercore to form a helical coil. v A wi er arin may then ride in a grooveadjacent the coil and pick off various levels of potential between thetwo ends of the coil.

Ditliculties arise in this particular method due to differences in thewire diameter, the coil diameter, the coil spacing, and thenon-uniformity of resistance of the wire itself.

These difliculties were overcome in a manner described in patentapplication Serial No. 357,155 filed May 2 5. 1953, for Method andApparatus for Winding Potentior'n ete'rs and Article Produced Thereby,by Curt I. Johnson. In the aforementioned patent application it wasproposed to wind the resistance Wire in the manner of a sprin By windinga spring of suitable resistance material wi a spring index ofapproximately six, and with sufficient initial tension so that thespring is closely wound, the spacing of each coil is directlyproportional to the mean diameter of the spring and inverselyproportional to the wire diameter. Therefore, the resistivity of thewound spring is a function of similar relationships.

The spring is formed by winding the wire on a mandrel into coils closelyadjacent each other. The angle of feed of the wire to the mandrel isadjusted so that it forms an acute angle with respect to the portion of"the spring already wound. Thus, an initial tension "can 'be placed inthe spring. The mandrel is removed and the spring is ready for use asthe potentiometer resistance element.

By placing the length of spring having a desired overall resistance on asuitable bobbin, a potentiometer of single or multiple turns can bewound on a winding form from the bobbin. The bobbin and the form to bewound are both driven and means is provided to vary the speed of thebobbin to permit a uniform resistance element to be wound on the form.This is accomplished by providing a master potentiometer having apick-off means, the arrangement being such that as said pick-off meansmoves from one end of the resistance element to the other, the variationin resistance is directly proportional, within certain tolerances, tothe amount of movement. Pick-off means is also provided closely adjacentthe point of application or said spring resistance element to saidwinding form. A potential is applied across said spring resistanceelement and said master potentiometer and said master potentiometer andsaid Winding form are driven in synchronism. The potentials at thepick-offs associated with said Winding form and said masterpotentiometer are compared and the difference "therebetween is used tovary the rate of rotation of the bobbin. Thus, the resistance of theresistance element on the winding form follows the same characteristicfunction as the reference potential.

With the above-described method and apparatus it is possible to wind apotentiometer, the resistance of which may be characteristically linearor non-linear, depending upon the resistance of the masterpotentiometer. In other words, the potentiometer produced is a precisecopy of the master potentiometer.

A difiicu'lty arises, however, in the use of this type of potentiometer.It will be seen, that if a load impedance is placedacross theoutputterminalsof the potentiometer, the potential at said terminals will notfollow the same characteristic curve, as the pick-off moves from one endof the resistance element to the other, as that followed by theresistance. As the value of impedance changes the variation of thepotential from said linear curve also varies. As the load impedance goesdown, the variation from linearity goes up, and vice versa.

To overcome the last-named difficulty is proposed to actually wind thespring resistance element on the winding form such that the resistance,from one end to the other, does not follow the same characteristic curveas the "potential from the master potentiometer. An impedance, such as aresistor, is placed between the pick-off means associated with thewinding form and some point on the spring resistance element, and theresistance element is then Wound on the form. When a load impedance, theequivalent of the aforementioned impedance, is connected to the pick-offand said one end, and the pick-off is moved from one end of theresistance element to the other, the voltage obtained follows the samecharacteristic curve as that followed by the resistance of said masterpotentiometer. In other words, if the resistance on said masterpotentiometer, from one end to the other, follows a linearcharacteristic curve, the output potential from the winding form, fromone end to the other, will also follow a linear characteristic curve.

For a more detailed description of the invention reference. is-made toFig. 1, which is an electromechanical schematic diagram of the windingapparatus, and Fig. 2, which is an electrical schematic of the circuitryinvolved. In the drawings, electrical wires are indicated by solid linesand mechanical shaft connections are indicated by dotted lines. Afterthe resistance wire has been wound on the mandrel so that the turns orcoils are in contact with each other the mandrel is removed. Sutficientinitial tension is used in the initial winding to assure accurate coilspacing. The length of coils appears as a helical spring.

It will be understood that while I have referred to the resistanceelement as being in the form of a series of turns or coils, thesub-portions could take other forms. It is but necessary that they be ofa form which permits elasticity or expansion, the term expansion beingused hereinafter to mean either increase in length or decrease inlength. Stated another way, the resistance element is of a constructionsuch that the resistance per unit length may be varied.

For a more detailed description of the manner of winding the resistancewire on the mandrel, reference is made to application Serial No.165,623, filed June 2, 1950, for Spring Coiling Device by Curt I.Johnson and James A. Nolrd, now Patent No. 2,650,638, issued September1, 953.

A length of straight wire is placed adjacent the length of coils. Oneend of the resistance element and one end of the straight wire aresecured to one end of the storage bobbin. The straight wire is then madetaut and the resistance element is expanded to separate adjacent coilsthereof along the length of the straight wire. When sufficient spacingis provided so as to prevent shorting between the coils the coils areWound on the bobbin.

The storage bobbin includes a cylindrical surface which is provided witha helical groove extending the length thereof. The resistance element isadapted to fit within the groove. The storage bobbin is shownschematically in Fig. 1 at 10. The end of the resistance element fixedto the bobbin is electrically connected to slip ring 11 associated withthe bobbin. The other end of the resistance element is connected to oneend of the potentiometer form or core illustrated by the referencenumeral 12, the details of which are shown in Figs. and 6. The slip ring13 is electrically connected to the last-named end of the resistanceelement. A D. C. potential is applied across the entire spring byconnecting to slip rings 11 and 13.

Once the spring resistance element is connected to the potentiometercore the straight wire may be removed from the bobbin.

A reference voltage potential is obtained by providing a masterpotentiometer illustrated by numeral 14. This master potentiometer ispreferably a potentiometer with the best possible degree of precision.It will be understood that other means could be provided as long as aprecision source of voltage is furnished to which the potentiometerbeing wound can be compared.

A drive motor 15 is excited by 115 volt 400 C. P. S. A. C. power supplyand is shaft connected through gear box 6 to a slip ring 17 associatedwith potentiometer 14. As schematically shown, slip ring 17 is securedto an arm 18 which is adapted to move around the periphery of thepotentiometer. A contact supporting member 19 is slidably mounted on arm18 and is adapted to follow a helical groove adjacent the helicalresistance winding of the potentiometer. A contact 20 is mounted onmember 19 and is adapted to ride on the potentiometer resistanceelement. The ends of this last-recited resistance element are connectedto a D. C. power supply so that a potential is placed thereacross.

Slip ring 17 is shaft connected to slip ring 13 and core 12. Both sliprings are connected to one side of a gear differential 21. Another sideof the differential is shaft connected to slip ring 11 and bobbin 10. Amotor generator servo unit 22 is shaft connected to the third side ofthe differential and is adapted to vary the drive speed of the bobbinwith respect to the core 12 in accordance with the rate at which thespring must be wound on the core to reproduce the master potentiometer.That is, the length of the spring can be varied in order to control thespacing of the coils which are wound on the core.

In order to control the motor generator servo unit 22 a contact 23 ismounted on a contact supporting member 24, said member 24 being slidablymounted on a fixed arm 25 adjacent core 12. A guide groove is pro videdadjacent the helical groove of the core for receiving a finger 26, shownin detail in Figs. 5 and 6, said finger being secured to member 24. Asthe core is rotated, finger 26 rides in the guide groove and causesmember 24 to move along arm 25. In this manner contact 23 engages thespring resistance element adjacent its point of application to the core.The spacing of the contact means with respect to the point at which theresistance element is applied to the core may be varied to provideoptimum correction for error in the distribution of the resistance inthe resistance element before it is fixed on the core. The lengthbetween the point of engagement by the contact means and the point wherethe element is fixed in relation to the core becomes the working lengthwhere the distribution is accurately controlled.

An impedance, such as resistor 90, is connected be tween the pick-offoutput terminal and the terminal of the resistance element which isconnected to the low side of the input potential. It should be madeclear that the impedance could just as well be connected be tween thepick-off output terminal and the terminal of the resistance elementconnected to the high side of the input potential. It is but necessarythat the impedance be connected to the same terminals which will laterbe used as output terminals to connect to the load impedance. A

Reference is now made to Fig. 2 along with Fig. 1. The spring resistanceelement being wound on core 12 is illustrated by numeral 27. The D. C.power supply places the same potential across resistance elements 14 and27. Contact 20, which picks off a potential from potentiometer 14, isconnected to ground potential. Terminal 28 of chopper 29 is alsogrounded. Arm 23 picks off a potential from resistance element 27 andsupplies this potential to terminal 30 of the aforementioned chopper.

Chopper 29 further includes an arm 31, said arm being pivoted at .oneend and.adapted -to contact terminals 28 or 30-at the other end thereof.A series connected coil 32 and capacitor 33 .are connected to an A. C.power supply of 115 v. 400 C. -P. S. Therefore, arm 31 is moved intocontact alternately with terminals 28 and 30 at the rate -.of 400cycles-per second.

In view *of the above it will be seen that an output signal will beproduced as 'long as there is a potential difference between terminals28 and 30. This output signal is RC coupled by resistor .34 andcapacitor 35 to transformer 36, the output of which is applied acrossaresistor 37. .An output signalis picked off the resistor at a desiredlevel and fed to a first amplifier 38 where the signal is amplified..The amplifier output signal is fed to a phase .adjustingnetwork 39 fromwhich it is fed to .servoamplifier 40.

Amplifier '40 maybe in theform of a conventional servoarnplifier of anumber of stages sufficient to obtain proper amplification. While thedetails of the amplifier do not form apart of the invention a schematiccircuit of an amplifier whichmay .be used is shown in Fig. 3. Briefly,the amp'lifierincludes a voltage amplifying input stage 41, the outputof which feeds across a transient bypass capacitor 42 and is .RC coupledto the control grid of a second voltage amplifying stage 4-3. The outputvoltage from stage 43 is.fed to aphase shifting net work 44 whichadvances the .phase of the signal 90. The output signal from network 44is supplied to one side of a duo-triode 45, the output of which issupplied through a voltage divider network 46 to the other side of theduoatriode. Thus, tube acts as a push-pul l amplifier stage. The outputsignals from tube 45 are 180 out of phase and are RC coupled to a secondduo-triode 47 winch acts as a push-pull power stage of amplification.'From the plates of tube 47 the output is limited by diode "58 andcoupled through transformer 48 to the control winding 49 of motor 50,said motor being a portion of servo -unit 22. Motor 50 is shaftconnected to the generator portion .51 of said unit. Both the motor andgenerator are of the two-phase type. A reference potential of 26 v. 400C. P. .S. A. C. is applied to the fixed windings 52 and 53 from thesystem power supply. The output winding 54 of the generator feeds backto the amplifier degeneratively for damping purposes. As shown in Fig.3, the generator output feeds to the control grid of tube 43 through aresistor 55. The phase difference between the feedback voltage from thegenerator and the input voltage supplied from amplifier 41 is 180. 7

The shaft output from servo unit v22 is connected to differential 21, asshown in .Fig. 1, so as to control the rate of rotation of storagebobbin 1%).

The overall mode of operation of the present invention will now bedescribed. A D. C. potential is applied across the master potentiometer14 and the resistance element 27 which is being wound on the core 12from storage bobbin 10. A drive motor 15 is connected to drive themaster potentiometer 14 and the core 12. Motor 15 drives storage bobbin10 through differential 21. As the resistance element is being wound onthe core a comparison is made between the potentials at contacts 20 and23 .to provide a signal which is a function of the errorin thedistribution of the resistance of the resistance element which is aboutto be wound on the form. If there is no error between the potentials nooutput signal is fed through the chopper and amplifiers to servo unit22. The shaft connecting the servo unit to the differential isstationary and therefore storage bobbin 10 and core 12 are rotated atthe same speed. However, if there is an error, which in the presentinvention appears as a potential difference between contacts 20 and 23,the difference potential exists between terminals 28 and 30. It will beseen that terminal 28 is placed at ground potential. If terminal 30 isabove ground potential the output voltage from the chopper will be ofone phase and if terminal 30iis-belowjground potential the outputvoltage from the chopper will be of the opposite phase. The phase of theoutput signal determines the direction of rotation of the shaft outputfrom servo unit 22 and the amplitude of the output signal determines therate of shaft rotation. Therefore, if too much of the resistance element27 has been traversed by the pick-off at a particular instant the shaftoutput from the servo unit, through the differential, decreases the rateof rotation of bobbin 10. If not enough of the resistance element 27 hasbeen traversed by said pick-off the servo unit shaf-t output is in theopposite direction and-increases the rate of rotation of bobbin 10. Theincrease or decrease in the rateof rotation of bobbin 10 increases ordecreases, respectively, the resistance of the resistance element perunit length. As the resistance per unit length changes the coil spacingis also changed. Therefore, the distribution of the resistance of theresistanceelement which is applied to the core is accurately controlledby the error signal.

Reference toiFig. '7 will clearly reveal the relationship between theresistance characteristic of a particular reference and the resistancecharacteristic of a plurality of potentiometers wound according -to theinstant invention, each of said plurality ofpotentiometers beingcompensated for different load impedances. In this instance theparticular reference source has a linear resistance characteristic butit will be understood that this has been chosen only by Way of example.The resistance curve for the reference is illustrated by numeral a.Curves b and -c represent the resistance characteristics for first andsecond potentiometers, respectively, which have been compensated fordifferent load impedances. The load impedance corrected for by theaforementioned first potentiometer is higher than corrected for by thesecond potentiometer. It is seen that the lower the load impedance thegreater the variation from the reference.

When'the load impedance for which the potentiometer is compensated iscoupled to the proper output terminals, the voltage output from thepotentiometer will follow the same characteristic curve as theresistance of the reference follows.

With the instant invention, variations of the output potential from adesired characteristic curve due to load impedance are effectivelycompensated for. When a customer desires a potentiometer for aparticular use, it is but necessary for him to specify the functionwhich the output 'voltageis to follow, the load impedance which is to beconnected to the output terminals, and which end of the potentiometer isto be used as one of the output terminals. The present apparatus is usedto wind a potentiometer, the resistance of which does not follow thedesired function, but when the output terminals are connected inaccordance with the customers specifications the output potential doesfollow said desired-function.

It will be obvious to those skilled in the art that the resistor doesnot have to be connected to one particular end of the resistanceelement. It could in fact be connected between any point on theresistance element and the pick-off in accordance with a customersspecifications. For example, the impedance could be connected between acenter tap on the resistance element and the pick-off in order toprovide a particular potential output.

Fig. '4 shows in a somewhat exaggerated manner the space relationshipbetween coils constructed from wire of varying diameter. For example,the wire of coils 60, '61 and '62 is smaller in diameter than the wireof coils 63, 64 and 65. It will be noted, however, that the spacometerfollow a linear characteristic curve when connected to a particular loadimpedance, the spacing of the coils will also vary in accordance withthe degree of vanation of the resistance from linearity.

In order that the correct spacing of the coils is always maintainedprovision is made for securing the coils to the winding form. One waywhich has shown good results is to mix an epoxy resin into a liquidstate and applying a thin film thereof to the helical groove into whichthe resistance element is to be placed. This is allowed to air dry untilit becomes tacky. This occurs toward the end of the pot life of theresin, and the winding operation is then performed. After the element iscompletely wound on the core, the complete unit is placed into an ovenof -80 C. and slowly rotated. The resin becomes soft and then hardens toa solid state. The hardened substance covers the lower portion of thecoil to impart additional rigidity thereto.

After the core has been completely wound it may then be assembled withthe remainder of the pentiometer. The complete assembly is shown inFigs. 5 and 6. The winding form or core 12 is fixedly mounted on thebase plate 66. The mechanical connection of the ends of the resistanceelement 27 is illustrated by numerals 67 and 68, said connections beingat opposite ends of the core and spaced 180 apart. The electricalconnections for the potentiometer are spaced 90 from the mechanicalconnections. As shown, a small wire 69 is led from a connection at oneend of the effective resistance via a longitudinally extending slot 70to a terminal post 71. A heavier wire 72 is connected to the terminalpost for connection to one side of the power supply which may be eitheran A. C. or D. C. source. A small wire 73 connects the other end of theeffective resistance and the terminal post 74. A heavier wire 75connects to the lastnamed post and extends through a longitudinallyextending aperture 76 to the other side of the aforementioned powersupply.

The contact supporting arm 77 is connected to one end of a member 79,said last-named member being secured centrally thereof to a shaft 80.Shaft extends through core 12 and base plate 66 and is journaledtherein. A second arm 78 is connected to the other end of member 79 andis adapted to counterbalance arm 77. The con tact 23 and its supportingmember 24 are similar to those shown in Fig. l and are illustrated bysimilar reference numerals. The finger 26 is adapted to ride in thehelical groove 81.

The electrical pick-off is through contact 23, supporting member 24, arm77 and member 79 to a slip ring 82. Slip ring 82 is secured to shaft 80along with member 79. A pair of wire contacts 83 are mounted on aterminal plate 84 and are adapted to engage slip ring 82 for picking offthe electrical potential thereon. A wire lead 85 is connected toterminal plate 84 and fed through aperture 76 to the other end of thecore. A slot 86 is provided in base plate 66 to permit lead-s 72, 75 and85 to be led from the potentiometer.

From the above it will be seen that a potential is applied to the coilsbetween the connection of wires 71 and 73. Rotation of arm 77 permitscontact 23 to pick off any potential between the points of appliedpotential.

The above-detailed description relates to a preferred embodiment of theinvention. It will be understood, however, that various elements areshown which serve only by way of example as a means for carrying out theinvention. The master potentiometer could be replaced by any kind ofvoltage supply which would supply a potential with which thepotentiometer being wound is to bear a relationship. For example, itwill be seen that a switchable decade box could be used. A source ofpotential could be applied across a resistor or a series of resistors,there being relay controlled means programmed to switch in and out smallincrements of resistance to provide a very accurate reference potential.While a D. C. error detecting network has been shown and described inthe preferred embodiment, an A. C. error detecting network could beprovided. The amount of initial tension which is placed in the springresistance element may be increased or decreased in accordance with therequirements of the situation. While a chopper has been shown as a meansfor obtaining a usable output signal from the voltage difference betweenthe potentiometer pick-off, it will be seen that other means could beprovided without departing from the scope of the invention. It is butnecessary that a signal be obtained which is a measure of the error ofone with respect to the other. It should be further understood thatpotentiometers other than linear potentiometers may be manufactured withthe present apparatus. By providing a particular non-linearpotentiometer or other voltage source as a reference the servomotorcontrol will regulate the rate of feed of the resistance spring to thecore. The only limitation comes in the amount the individual coils canbe spaced. That is, they cannot be so close together as to provide ashort circuit or so far apart as to prevent engagement between at leastone of the coils and the contact means. Too great a spacing results inan open circuit.

While there have been shown and described and pointed out thefundamental novel features of the invention as applied to a preferredembodiment, it will be understood that various omissions andsubstitutions and changes in the form and details of the deviceillustrated and in its operation may be made by those skilled in theart, without departing from the spirit of the invention. It is theintention, therefore, to be limited only as indicated by the scope ofthe following claims.

What is claimed is:

1. In an apparatus for producing potentiometers, a resistance elementwhose resistance per unit length may be varied, contact means fortraversing the length of a portion of said resistance element, means formaking substantially all of the part of said portion traversed by saidcontact means rigid so that the distribution of the resist ance of saidrigid part may not be varied, means for applying a potential across saidportion, impedance means connected to said contact means and to a fixedpoint on said rigid part, a source of reference potential which variesaccording to a prescribed function, means for detecting the errorbetween the potential at said contact means and said reference potentialand providing a signal which is a function thereof, and means responsiveto said signal for controlling the distribution of the resistance ofsaid portion which has not become rigid.

2. In an apparatus for producing potentiometers, a resistance element inthe form of a series of elastic subportions, said resistance elementhaving a potential applied across a portion thereof, contact means,means mounting said contact means for traversing along the length ofsaid portion from one end of said portion to the other, means forapplying a part of said portion which has been traversed by said contactmeans to a form so that the distribution of the resistance of said partis substantially fixed, impedance means connected between a fixed pointon-said portion and said contact means, a source of variable referencepotential, means for providing a signal which is a function of the errorbetween the voltage at said contact means and said reference potential,and means responsive to said signal for determining the distribution ofsaid elastic sub-portions before they are fixed to said form.

3. In an apparatus for winding potentiometers, a resistance element,said resistance element being in the form of a series of turns thespacing of which may be varied, contact means, means mounting saidcontact means for movement over said turns from one end of saidresistance element to the other, means for mounting a portion of saidresistance element traversed by said contact means on a form in a mannerto make substantially all of the turns of said portion in fixed spaced 9relation, impedance means connected between a point near one end of saidresistance element and said contact means so that it is in parallel withthe portion of said resistance element between the last-named one endand said contact means, a reference potential which varies in accordancewith a function, means for comparing the potential at said contact meanswith said reference potential and providing a signal which is a functionof the difference therebetween, and control means responsive to saidsignal for determining the spacing of the turns of said resistanceelement as they are applied to said form.

4. An apparatus for winding otentiometers which are adapted to beconnected to a predetermined load impedance and whose voltage outputwhen connected to said load impedance will follow a predeterminedfunction, comprising a resistance element whose length may be varied,contact means mounted for movement along a portion of said resistanceelement from one end of said portion to the other, means for supplying apotential across said portion, means for applying a substantial part ofsaid portion traversed by said contact means to a form so that thedistribution of the resistance of said part is substantially fixed,impedance means, said impedance means being substantially equivalent invalue to said load impedance and connected to the same points of saidportion to which the load impedance is adapted to be connected, a sourceof reference potential which follows said predetermined function, meansfor comparing the potential at said contact means with said referencepotential and providing a signal which is a function of the errortherebetween, and means responsive to said signal for determining thedistribution of the resistance of said portion before it becomes fixedto said form.

5. An apparatus for winding otentiometers which are adapted to beconnected to a predetermined load impedance and whose voltage outputwhen connected to said load impedance will follow a predeterminedfunction, comprising a resistance element having a plurality ofsubportions which are of the same general configuration, contact meansmounted for movement along a length of said resistance element from oneend of said length to the other, means for supplying a potential acrosssaid length, means for applying a substantial part of said lengthtraversed by said contact means to a form so that the subportionsthereof bear a fixed relation to each other, impedance means, saidimpedance means being substantially equivalent in value to said loadimpedance and connected to the same points of said length to which theload impedance is adapted to be connected, a source of referencepotential which varies according to a predetermined function, means forcomparing the potential at said contact means with said referencepotential and providing a signal proportional to the error therebetween,and means responsive to said signal for controlling the rate at whichthe sub-portions are applied to said form.

References Cited in the file of this patent UNITED STATES PATENTS2,590,246 Heckman et al Mar. 25, 1952 2,620,990 Cary et a1. Dec. 9, 19522,639,864 Hale May 26, 1953 2,643,068 Harris June 23, 1953 2,645,429Scott et al. July 14, 1953 2,653,772 Scott Sept. 29, 1953

